KHIZOPODA. 



35 



All the Polycystina are microscopic, and they are all inhabitants of 

 the sea. They are best known to students of the microscope as 

 the " Fossil Infusoria of Barba- 

 does " (fig. 14), as they occur in 

 incalculable numbers in a sand- 

 stone in that island. 



They are, however, in no way 

 related to the "Infusoria" pro- 

 perly so Ciilled, having much 

 stronger affinities with the For- 

 aminifera. We know, also, now, 

 that deposits of the flinty shells 

 of these beautiful microscopic 

 organisms are in process of for- 

 mation in our oceans at great 

 depths. 



In the third family {CoUozoa) 

 are included a number of singu- 

 lar gelatinous organisms which 



may be as large as an ordinary marble, but are often hardly visible 

 to the unassisted eye. They are found floating passively at the sur- 

 face of most seas ; and they may either have no skeleton, or may 

 secrete a flinty framework, which may give rise to a fenestrated 



Fig. 14.— Shells of Polycystina from the 

 " Barhadoes Earth," greatly magnified. 

 (Original.) 



Fig. 1.5. 



-a Siliceous shell of CoUosphcera ; b SpJic^ozoum, showing the radiating 

 pseudopodia and groups of siliceous spicnla (after Miiller). 



shelf, as in Collosphcera (fig. 15, a), or may remain in the foi-m of 

 scattered spicules, as in Splicerozoum (fig. 15, h). In Tkalasdcolla 

 the skeleton is wanting. 



A fourth family has been constituted under the name of Heliozoa, 

 for a number of singular and beautiful microscopic organisms, which 

 mostly occur in fresh water, and which cannot be regarded as typical 

 Radiolarians, though more closely allied to this group of the Pro- 



